J. C. Branner — Geology of Fernando de Nor onha. 159 



The Portao beds of tuff (ISTo. 54) are more homogeneous than 

 those at Capim Azul. The individual fragments into which it 

 breaks upon disintegration are seldom more than two inches in 

 diameter, and the weathered surface of the beds has a lumpy 

 rough appearance. The beds are regularly stratified, the dark 

 brown material being streaked with lighter and darker bands. 

 They dip southwest and southeast at an angle of nearly 45°, 

 the opening being cut in a kind of syncline whose axis dips to 

 the south. Overlying the tuff is a bed of hard rock containing 

 many rectangular crystals, specimens of which unfortunately 

 have not been preserved. This hard but much jointed rock 

 fills the little depression, or syncline, in the tuff, and forms a 

 nearly horizontal roof for this natural tunnel. The triangular 

 gap between the tuff and its overlying bed is filled with irreg- 

 ularly stratified fragments which could not be examined. 



The rock walls of the Portao from one face to the other are 

 a little less than one hundred feet in thickness ; the roof is 

 about forty feet above the water at mean tide, and the open- 

 ing is about forty feet in width. At the time of my visit the 

 water did not have free passage through the opening, the 

 northern entrance being barred by a narrow dyke of very hard 

 basalt, about fifteen feet high, traversing the tuff and standing 

 nearly square across its front. 



The process by which the great opening has been made 

 beneath this isthmus is not without interest. The surf about 

 the Sapato is always violent, and especially so upon its south 

 side. The excavation has all been done upon this south side, 

 the character and clip of the rocks contributing largely to the 

 result. If the waves breaking against the southwest dip of the 

 excavated rock were carried up its slope, they were promptly 

 checked by the hard overlying rock which forms the tunnel's 

 roof. When, in the course of time, the wall was pierced, the 

 waves struck the small basaltic dyke referred to above and this 

 has ever since barred their progress. A gap, however, has 

 been made through this dyke where it receives the full force 

 of the waves. The dip of the dyke is toward the south and 

 when the incoming waves plunge through the opening they 

 strike this wall and are thrown off at a tangent, leap high in 

 the air and fall in overwhelming volume and in spray upon the 

 shingle of the northern beach. The swells and less violent 

 waves occasionally lift their great volumes of water and pour 

 intermittent cataracts over the gap on the little dyke, and 

 down the channel which is opened to the left through the tuff 

 between the wall and the overhanging cliffs. 



The lowest beds exposed at the extreme western end of the 

 Sapato are of basaltic tuff. The rock is reddish brown, soft and 

 somewhat granular and contains an abundance of included frag- 



